1. Field of the Invention
This invention relates to a method for driving a nonvolatile semiconductor memory device based on transistor type memory cells having a floating electrode.
2. Background Art
With the recent progress of downscaling, a NAND flash memory having a floating gate has an increasing need to decrease an electrical film thickness of an insulating film bearing charge retention. This raises a problem of releasing charge stored in a floating gate by decreasing the electrical film thickness to cause degradation of retention characteristics of threshold voltage. Moreover, there is a tendency to use a high dielectric material as a part of the insulating film with the purpose of decreasing the electrical film thickness of the insulating film. The high dielectric material is highly defective and has characteristics trapping charges. Charges trapped by the high dielectric material affect the threshold voltage, and thus release of trapped charges with time results in the degradation of the retention characteristics of the threshold voltage.
To avoid problems specifically arising in such a floating gate flash memory having the high dielectric material, a method for injecting charges favorable to a charge trapping characteristics of the insulating film has been investigated.
For example, US Patent Application Publication No. 2005/0006698 discloses a technology enhancing charge retention characteristics and improving the reliability by performing a program operation on all memory cells existing in an erase unit before applying an erase bias, thereafter applying the erase bias.
JP-A 2007-35214 (Kokai) discloses a technology assuring a read margin to improve the reliability of the operation by applying a lower voltage than a semiconductor substrate to a control gate and de-trapping charges trapped near an oxide film or an interface between an oxide film and the semiconductor substrate.
On the other hand, also in a MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) nonvolatile memory device, it has been possible to enhance the retention characteristic and speed up a program and erase operation by controlling positions of charges in the insulating film.
However, in a floating gate flash memory, the enough investigation has not been performed about performance enhancement by control of charge trapping in the insulating film, particularly a block insulating film, and a method for driving necessary for enhancement of the retention characteristics focusing attention on the block insulating film and speed up of the program and erase operation has not been proposed.